Heretofore, compound semiconductor elements have been mainly used to process signals at high frequency band (microwave band). In recent years, however, laterally diffused metal oxide semiconductor (LDMOS) transistors formed in semiconductor substrates have been used instead of these compound semiconductor elements. The LDMOS transistors are advantageous in that the LDMOS transistors can be fabricated at lower costs than the compound semiconductor elements. In addition, the LDMOS transistors are also advantageous in that the breakdown voltage thereof can be enhanced relatively easily. Semiconductor devices (integrated circuits) each internally including an LDMOS transistor are widely used in mobile phones, wireless LAN equipment, in-vehicle electronic devices, and the like.
In addition, there are many semiconductor devices each including an electrostatic discharge (ESD) protection element in a chip to prevent an electrostatic breakdown of the device. There are also developed thyristor-type ESD protection elements achieving high discharge performance with almost the same structure as LDMOS transistors.    Patent Document 1: U.S. Pat. No. 5,903,032    Patent Document 2: U.S. Pat. No. 6,144,070    Patent Document 3: Japanese Laid-open Patent Application Publication No. 2001-320047    Patent Document 4: Japanese Laid-open Patent Application Publication No. 2002-94063